This invention relates to a digital multifrequency signaling receiving system using discrete Fourier transform and more specifically to a digital multifrequency signaling receiving system with reduced number of operations.
Presently, multifrequency signaling (hereinafter referred to as MF signaling) is used for transmission of inter-office register signaling as standardized signaling between crossbar exchanges. For the exchanges themselves, stored-program controlled exchanges and time division multiplexed exchanges have come to be used, and for the signaling system, there is a movement to swtich over to the common channel signaling system. These stored-program controlled exchanges, however, must employ the MF signaling for the crossbar exchanges, so that it is important to improve receiving systems for such MF signaling.
In such MF signaling receiving systems, the input frequency has conventionally been detected by the digital filter (DF) method. This DF method detects the frequency of the input signal through filter banks. For technical literature, reference is made to "An All Digital Telephony Signalling Module" (IEEE Proc. circuit and system theory, 1975), by P. Kaul and H. Lieberman.
According to the DF method, however, filters for an analogue receiver are directly replaced by the digital filters, so that the order of the filters becomes large and the size of hardware used or number of operations will be increased.
As another conventional method, there is the discrete Fourier transform (DFT) method. In this method, the input signal is Fourier-transformed, and the input frequency is detected by obtaining the coefficients of the Fourier series of the input signal. For technical literature concerning this method, reference is made to "Digital MF Receiver using Discrete Fourier Transform" (IEEE Trans. on Communications vol. COM-21, No. 12).
However, as may be seen from the above literature, software in a universal computer cannot process the input signal in real time in the DFT MF reception system, so that the receiver need be composed of dedicated hardware. Although such receiver may effectively be used as a receiver for large-office service, it is defective in economical efficiency, extension cost, etc. for small-office use.